Semiconductor device including a MOSFET and a Schottky junction

ABSTRACT

A semiconductor device, including a MOSFET, has a plurality of transistor cell regions disposed in a semiconductor substrate. A source electrode of the MOSFET is disposed over a main surface of the semiconductor substrate and is in contact with a top surface of a source region in each of the plurality of transistor cell regions. A drain electrode of the MOSFET is a disposed over a back surface of the semiconductor substrate and is electrically connected to the semiconductor substrate. A Schottky cell region is disposed between the plurality of transistor cell regions in the semiconductor substrate. The source electrode is in contact with a part of the main surface of the semiconductor so as to form a Schottky junction in the Schottky cell region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/404,285filed Mar. 14, 2009 now U.S. Pat. No. 7,829,946, which is a continuationof application Ser. No. 11/482,765 filed Jul. 10, 2006 (now U.S. Pat.No. 7,518,208 issued Apr. 14, 2009), which is a continuation ofapplication Ser. No. 10/948,305 filed Sep. 24, 2004 (now U.S. Pat. No.7,078,782 issued Jul. 18, 2006), which is a continuation of applicationSer. No. 10/269,970 filed Oct. 15, 2002 (now U.S. Pat. No. 6,806,548issued Oct. 19, 2004).

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device and particularlyto a technique which is effectively applicable to a semiconductor devicehaving a power transistor and a Schottky barrier diode (SBD) on one andthe same semiconductor substrate.

As a semiconductor device used as a switching device in a poweramplifier or a power supply circuit there is known, for example, a powertransistor called power MISFET (Metal Insulator Semiconductor FieldEffect Transistor). The power MISFET has a structure wherein pluraltransistor cells comprising fine patterns of MISFETs are connected inparallel to obtain a large power. Power MISFETs called vertical type andhorizontal type are known. As to the vertical type, one called a trenchgate structure is also known.

MISFET indicates an insulated gate type field effect transistor whereina gate insulating film (insulating film) is interposed between a channelforming region (semiconductor) and a gate electrode. One wherein thegate insulating film is formed by a silicon oxide film is generallycalled MOSFET (Metal Oxide Semiconductor Field Effect Transistor).Moreover, one wherein an electric current flows in the thickness (depth)direction of a semiconductor substrate is called a vertical type, whileone wherein an electric current flows in the surface direction of asemiconductor substrate is called a horizontal type. Further, one havinga channel (conductive passage) of electrons in a channel forming regionbetween source and drain regions (i.e., under a gate electrode) iscalled n type (or n-channel conductor type), and one having a channel ofholes is called p type (or p-channel conductive type). The trench gatestructure indicates a gate structure wherein in the interior of a trenchformed in one main surface of a semiconductor substrate there is formeda gate electrode through a gate insulating film. As to the power MISFETof the trench gate structure, it is described in Japanese PublishedUnexamined Patent Application No. Hei 7(1995)-249770 for example.

FIG. 19 is a circuit diagram of a conventional synchronous rectificationtype DC/DC converter using power MISFETs and FIG. 20 is a timing chartof a power MISFET for main switch and a power MISFET for synchronousrectification both shown in FIG. 19. In FIG. 19, Q1 denotes a powerMISFET for main switch, Q2 denotes a power MISFET for synchronousrectification, BD1 and BD2 denote body diodes, and SBD denotes aSchottky barrier diode. The body diodes BD1 and BD2 are incorporated inthe power MISFETs respectively and are connected in parallel with thepower MISFETs. The Schottky barrier diode SBD is connected in parallelwith the power MISFET Q2 for synchronous rectification.

In the synchronous rectification type DC/DC converter shown in FIG. 19,a period called “Dead time” is set as shown in FIG. 20 so as to preventa lead-through current caused by simultaneous turning ON of both Q1 andQ2. In this period there flow an electric current like B in FIG. 19. Inthis case, a circuit loss can be decreased by connecting a Schottkybarrier diode smaller in forward voltage (VF) than the body diode BD2 inparallel with the power MISFET Q2 for synchronous rectification.

The use of the Schottky barrier diode is essential in such a circuit. Inthis connection, a semiconductor device is disclosed, for example, inJapanese Published Unexamined Patent Application No. Hei 10(1998)-150140wherein a semiconductor chip with a power MISFET mounted thereon and asemiconductor chip with a Schottky barrier diode mounted thereon aresealed with a single seal member. Further, a semiconductor device withboth a power MISFET of the trench gate structure and a Schottky barrierdiode mounted on a single semiconductor substrate is disclosed, forexample, in Japanese Published Unexamined Patent Application No. Hei11(1999)-154748.

SUMMARY OF THE INVENTION

In the semiconductor device wherein a semiconductor chip with a powerMISFET mounted thereon and a semiconductor chip with a Schottky barrierdiode mounted thereon are sealed with a single seal member, an electricconnection between the power MISFET and the Schottky barrier diode ismade using a bonding wire, so a parasitic inductance increases and thecircuit efficiency of a DC/DC converter, etc. is deteriorated.

On the other hand, in the semiconductor device with both a power MISFETof the trench gate structure and a Schottky barrier diode mounted on asingle semiconductor substrate, the bonding wire for electric connectionbetween the power MISFET and the Schottky barrier diode can be omitted,so it is possible to decrease a parasitic inductance. As a result, it ispossible to control the current flowing time in the body diode of thepower MISFET and hence possible to greatly decrease the loss of “Deadtime” period during operation of the DC/DC converter which is controlledby PWM.

However, having studied the semiconductor device with both a powerMISFET of the trench gate structure and a Schottky barrier diode mountedon one and the same semiconductor substrate, the inventor in the presentcase found out the following problems.

According to the structure of a conventional semiconductor device,plural cells are defined by trenches in one main surface of asemiconductor substrate, and out of the plural cells, transistor cellsfor the formation of MISFETSs of the trench structure and Schottky cellsfor the formation of Schottky barrier diodes are arranged in analternate manner. The width of each Schottky cell and that of eachtransistor cell arc of the same size. If transistor cells and Schottkycells are arranged in an alternate manner, it is necessary that thetrench positioned between adjacent transistor cell and Schottky cell bepresent in a number corresponding to the number of Schottky cells, thusresulting in an increase in a plane size of the semiconductor substrate,i.e., an increase in chip size.

In the transistor device having both power MISFET of the trench gatestructure and Schottky barrier diode on one and the same semiconductorsubstrate, the area of the power MISFET and that of the Schottky barrierdiode are determined so as to satisfy characteristics of the powerMISFET and VF characteristics of the Schottky barrier diode, which arerequired by a user. Therefore, it is necessary that the ratio betweenthe area of the power MISFET and that of the Schottky barrier diode bevaried freely so as to meet the user's needs, i.e., it is necessary toensure the freedom of design.

When the power MISFET is OFF, in the bottom portion of the trench formedbetween a transistor cell and the associated Schottky cell, and on thetransistor cell side, there spreads a depletion layer based on pnjunction between a drain region and a channel forming region, so thatvoltage is not directly applied to the gate insulating film. In contrasttherewith, in the trench portion adjacent to the Schottky cell, there isnot formed a depletion layer based on pn junction, so that voltage isapplied directly to the gate insulating film, with consequent loweringin gate breakdown voltage of the power MISFET.

The Schottky barrier diode is formed by bonding a metal electrode to asemiconductor. But because of electric field concentration at an end ofthe metal bonded portion, there occurs a lowering in breakdown voltageof the Schottky barrier diode.

It is an object of the present invention to provide a technique capableof attaining the reduction in size of a semiconductor device which has apower transistor and a Schottky barrier diode on one and the samesemiconductor substrate.

It is another object of the present invention to provide a techniquecapable of ensuring the freedom of design in a semiconductor devicewhich has a power transistor and a Schottky barrier diode on one and thesame semiconductor substrate.

It is a further object of the present invention to provide a techniquecapable of enhancing the breakdown voltage of a Schottky barrier diodein a semiconductor device which has a power transistor and the Schottkybarrier diode on one and the same semiconductor substrate.

It is a still further object of the present invention to provide atechnique capable of enhancing the breakdown voltage between source anddrain of a power transistor in a semiconductor device which has thepower transistor and a Schottky barrier diode on one and the samesemiconductor device.

The above and other objects and novel features of the present inventionwill become apparent from the following description and the accompanyingdrawings.

Typical inventions disclosed herein will be outlined below.

(1) A semiconductor device according to the present invention comprises:

a first region and a second region formed on a main surface of asemiconductor substrate;

a plurality of first conductors and a plurality of second conductorsformed in the first and second regions respectively;

a first semiconductor region and a second semiconductor region formedbetween adjacent said first conductors in the first region, the secondsemiconductor region lying in the first semiconductor region and havinga conductivity type opposite to that of the first semiconductor region;

a third semiconductor region formed between adjacent said secondconductors in the second region, the third semiconductor region havingthe same conductivity type as that of the second semiconductor regionand being lower in density than the second semiconductor region; and

a metal formed on the semiconductor substrate in the second region,

the third semiconductor region having a metal contact region for contactwith the metal,

the metal being electrically connected to the second semiconductorregion, and

a center-to-center distance between adjacent said first conductors inthe first region being smaller than that between adjacent said secondconductors in the second region.

(2) In the semiconductor device described in the above means (1),

the first and second conductors are formed through an insulating filmwithin trenches formed in the semiconductor substrate;

the third semiconductor region is formed also under the firstsemiconductor region in the first region; and

a MISFET is formed in the first region, the MISFET comprising the firstconductors, the second semiconductor region, and the third semiconductorregion as gate, source, and drain, respectively.

(3) In the semiconductor device described in the above means (2),

a center-to-center distance between adjacent said second conductors inthe second region is larger than the depth of each of the trenches in aplane perpendicular to the semiconductor substrate.

(4) In the semiconductor device described in the above means (2),

a fourth semiconductor region is included in the third semiconductorregion in the second region, the fourth semiconductor region beingformed so as to surround an end portion of the metal contact region andthe second conductors and having a conductivity type opposite to that ofthe third semiconductor region.

(5) In the semiconductor device described in the above means (4),

the fourth semiconductor region is a guard ring.

(6) In the semiconductor device described in the above means (4),

a center-to-center distance between adjacent said second conductors inthe second region is not smaller than twice the center-to-centerdistance between adjacent said first conductors in the first region.

(7) In the semiconductor device described in the above means (1),

the metal in the second region and the third semiconductor region form aSchottky junction.

(8) In the semiconductor device described in the above means (1),

the first region and the second region are adjacent to each other andare each formed in a plural number.

(9) In the semiconductor device described in the above means (1),

the first region and the second region are adjacent to each other, andthe first region is formed in a plural number, while the second regionis formed in a singular number.

(10) A semiconductor device according to the present inventioncomprises:

a first region and a second region formed on a main surface of asemiconductor substrate;

a plurality of first conductors and a plurality of second conductorsformed in the first and second regions respectively;

a first semiconductor region and a second semiconductor region formedbetween adjacent said first conductors in the first region, the secondsemiconductor region lying in the first semiconductor region and havinga conductivity type opposite to that of the first semiconductor region;

a third semiconductor region formed between adjacent said secondconductors in the second region, the third semiconductor region havingthe same conductivity type as that of the second semiconductor regionand being lower in density than the second semiconductor region; and

a metal formed on the semiconductor substrate in the second region,

the third semiconductor region having a metal contact region for contactwith the metal,

the metal being electrically connected to the second semiconductorregion, and

a fourth semiconductor region being included in the third semiconductorregion in the second region, the fourth semiconductor region beingformed so as to surround an end portion of the metal contact region andthe second conductors and having a conductivity type opposite to that ofthe third semiconductor region.

(11) In the semiconductor device described in the above means (10),

the third semiconductor region is formed also under the semiconductorregion in the first region;

a first insulating film and a second insulating film are formedrespectively between the first conductor and the semiconductor substrateand between the second conductor and the semiconductor substrate; and

a MISFET is formed in the first region, the MISFET comprising the firstconductors, the second semiconductor region, and the third semiconductorregion as gate, source, and drain, respectively.

(12) In the semiconductor device described in the above means (11),

the first and second conductors are formed through the first and secondinsulating films within trenches formed in the semiconductor substrate.

(13) In the semiconductor device described in the above means (11),

the metal in the second region and the third semiconductor region form aSchottky junction.

(14) In the semiconductor device described in the above means (11),

the depth of the fourth semiconductor region in a plane perpendicular tothe semiconductor substrate is larger than the depth of the firstsemiconductor region.

(15) In the semiconductor device described in the above means (11),

a third insulating film thicker than the first and second insulatingfilms is formed in a region formed on the main surface of thesemiconductor substrate in the second region and including an end faceof the metal contact region.

(16) A semiconductor device according to the present inventioncomprises:

a first region and a second region formed on a main surface of asemiconductor substrate;

a plurality of first conductors and a plurality of second conductorsformed in the first and second regions respectively;

a first semiconductor region and a second semiconductor region formedbetween adjacent said first conductors in the first region, the secondsemiconductor region lying in the first semiconductor region and havinga conductivity type opposite to that of the first semiconductor region;

a third semiconductor region formed between adjacent said secondconductors in the second region, the third semiconductor region havingthe same conductivity type as that of the second semiconductor regionand being lower in density than the second semiconductor region;

a fourth semiconductor region having the same conductivity type as thatof the third semiconductor region and higher in density than the thirdsemiconductor region, formed under the third semiconductor region; and

a metal formed on the semiconductor substrate in the second region,

the metal being electrically connected to the second semiconductorregion, and

the third semiconductor region being in contact with the metal to form aSchottky junction.

(17) A semiconductor device according to the present inventioncomprises:

a first region and a second region formed on a main surface of asemiconductor substrate;

a plurality of first conductors and a plurality of second conductorsformed in the first and second regions respectively;

a first semiconductor region and a second semiconductor region formedbetween adjacent said first conductors in the first region, the secondsemiconductor region lying in the first semiconductor region and havinga conductivity type opposite to that of the first semiconductor region;

a first semiconductor region formed between adjacent said secondconductors in the second region, the third semiconductor region havingthe same conductivity type as that of the second semiconductor regionand being lower in density than the second semiconductor region; and

a metal formed on the semiconductor substrate in the second region,

the metal being electrically connected to the second semiconductorregion,

the third semiconductor region being in contact with the metal to form aSchottky junction, and

the first and second regions being adjacent to each other, and thesecond region being formed so as to surround the first region in a planeparallel to the semiconductor substrate.

(18) A semiconductor device according to the present inventioncomprises:

a first region and a second region formed on a main surface of asemiconductor substrate;

a plurality of first conductors and a plurality of second conductorsformed in the first and second regions respectively;

a first semiconductor region and a second semiconductor region formedbetween adjacent said first conductors in the first region, the secondsemiconductor region lying in the first semiconductor region and havinga conductivity type opposite to that of the first semiconductor region;

a third semiconductor region formed between adjacent said secondconductors in the second region, the third semiconductor region havingthe same conductivity type as that of the second semiconductor regionand being lower in density than the second semiconductor region;

a first metal and a second metal formed on the semiconductor substratein the first and second regions respectively,

the first metal being electrically connected to the second semiconductorregion;

the second metal being in contact with the third semiconductor region toform a Schottky junction,

the first metal and the second metal being connected togetherelectrically, and

a work function of the second metal being larger than that of the firstmetal.

(19) A semiconductor device according to the present inventioncomprises:

a first region and a second region formed on a main surface of asemiconductor substrate;

a plurality of first conductors and a plurality of second conductorsformed in the first and second regions respectively;

a first semiconductor region and a second semiconductor region formedbetween adjacent said first conductors in the first region, the secondsemiconductor region lying in the first semiconductor region and havinga conductivity type opposite to that of the first semiconductor region;

a third semiconductor region formed between adjacent said secondconductors in the second region, the third semiconductor region havingthe same conductivity type as that of the second semiconductor regionand being lower in density than the second semiconductor region;

a fourth semiconductor region having the same conductivity type as thatof the third semiconductor region and higher in density than the thirdsemiconductor region, formed under the first semiconductor region in thefirst region; and

a metal formed on the semiconductor substrate in the second region,

the metal being electrically connected to the second semiconductorregion; and

the metal being in contact with the third semiconductor region to form aSchottky junction.

(20) In the semiconductor device described in the above means (19),

the first and second conductors are formed through an insulating filmwithin trenches formed in the semiconductor substrate; and

in the first region is formed a MISEET comprising the first conductors,the second semiconductor region, and the fourth semiconductor region asgate, source, and drain, respectively.

(21) In the semiconductor device described in the above means (19),

a fourth semiconductor region is included in the third semiconductorregion in the second region, the fourth semiconductor region beingformed so as to surround an end portion of the Schottky junction andhaving a conductivity type opposite to that of the third semiconductorregion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit diagram of a semiconductor deviceaccording to a first embodiment of the present invention;

FIG. 2 is chip layout diagram showing a schematic construction of thesemiconductor device illustrated in FIG. 1;

FIG. 3 is a partially enlarged, schematic plan view of FIG. 2;

FIG. 4 is a schematic sectional view taken along line A-A in FIG. 3;

FIG. 5 is a chip layout diagram showing a schematic construction of asemiconductor device according to a second embodiment of the presentinvention;

FIG. 6 is an enlarged, schematic plan view of region C which is a partof FIG. 5;

FIG. 7 is an enlarged, schematic plan view of region D which is a partof FIG. 5;

FIG. 8 is a schematic sectional view taken along line B-B in FIG. 5,with an intermediate portion omitted;

FIG. 9 is a schematic sectional view taken along line C-C in FIG. 5,with an intermediate portion omitted;

FIG. 10 is a partially enlarged, schematic sectional view of FIG. 8;

FIG. 11 is a partially enlarged, schematic sectional View of FIG. 9;

FIG. 12 is a chip layout diagram showing a schematic construction of asemiconductor device according to a third embodiment of the presentinvention;

FIG. 13 is a schematic sectional view showing a schematic constructionof a semiconductor device according to a fourth embodiment of thepresent invention;

FIG. 14 is a schematic sectional view showing a schematic constructionof a semiconductor device according to a fifth embodiment of the presentinvention;

FIG. 15 is a schematic sectional view showing a schematic constructionof a semiconductor device according to a sixth embodiment of the presentinvention;

FIG. 16 is a schematic sectional view showing a schematic constructionof a semiconductor device according to a seventh embodiment of thepresent invention;

FIG. 17 is a schematic sectional view showing a schematic constructionof a semiconductor device according to an eighth embodiment of thepresent invention;

FIG. 18 is a schematic sectional view showing a schematic constructionof a semiconductor device according to a ninth embodiment of the presentinvention;

FIG. 19 is a circuit diagram of a conventional synchronous rectificationtype DC/DC converter; and

FIG. 20 is a timing chart of a power MISFET for main switch and a powerMISFET for synchronous rectification both shown in FIG. 19.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detailhereinunder with reference to the accompanying drawings. In all thedrawings for explaining the embodiments of the present invention,components having the same functions are identified by like referencenumerals, and repeated explanations thereof will be omitted.

First Embodiment

In this embodiment the present invention is applied to a semiconductordevice which has a power MISFET of a trench gate structure and aSchottky barrier diode on one and the same semiconductor substrate.

FIG. 1 is an equivalent circuit diagram of the semiconductor device ofthe first embodiment;

FIG. 2 is a chip layout diagram showing a schematic construction of thesemiconductor device illustrated in FIG. 1;

FIG. 3 is a partially enlarged, schematic plan view of FIG. 2; and

FIG. 4 is a schematic sectional view taken along line A-A in FIG. 3.

As shown in FIG. 1, the semiconductor device of this embodiment has apower MISFET Q, a body diode BD, and a Schottky barrier diode SBD. Theseelements (Q, BD, SBD) are mounted on one and the same semiconductorsubstrate. The body diode BD and the Schottky barrier diode SBD areconnected in parallel with the power MISFET Q. In the body diode BD andthe Schottky barrier diode SBD, respective cathode regions are connectedto a drain region of the power MISFET Q, while respective anode regionsare connected to a source region of the power MISFET Q. The power MISFETis of a structure wherein plural transistor cells comprising finepatterns of MISFETS are connected in parallel to obtain a large electricpower.

As shown in FIGS. 2 to 4, the semiconductor device of this embodiment isconstituted mainly by a semiconductor substrate 1. As the semiconductorsubstrate 1 there is used, for example, a semiconductor substratecomprising an n⁺ type semiconductor layer 1 a formed of a single crystalsilicon and an n⁻ type semiconductor layer 1 b formed of a singlecrystal silicon on one main surface of the n⁺ type semiconductor layer 1a. An electrode 16 is formed on another main surface (back side) opposedto the one main surface of the semiconductor substrate 1. The electrode16 is used as a drain electrode and, for example, it is formed of aconductive material containing gold (Au) as a main component.

In a central portion 5M surrounded with a peripheral portion 1N of theone main surface of the Semiconductor substrate 1 there is provided acell array portion which comprises plural cells defined by trenches 5.Cells selected from among the plural cells are Schottky cells 20A withSchottky barrier diodes formed therein, and the other cells, exclusiveof the Schottky cells 20A, are transistor cells 20B with MISFETs formedtherein, the MISFETs using conductors as gate electrodes which areformed within the trenches 5 through gate insulating films 6. In thisembodiment, the plural cells, including the Schottky cells 20A and thetransistor cells 20B, are formed in a stripe shape extending in a seconddirection (Y direction). In the plural cells, one Schottky cell 20A andtwo transistor cells 20B are arranged alternately in a first direction(X direction) which is orthogonal to the second direction.

The MISFET in each transistor cell 20B, as shown in FIG. 4, mainlycomprises a channel forming region, a gate insulating film 6, a gateelectrode 7, a source region, and a drain region. The channel formingregion is constituted by a p⁻ type semiconductor region (well region) 4formed on a main surface of the n⁻ type semiconductor layer 1 b. Thegate insulating film 6 is formed along inner walls of the associatedtrench 5 and, for example, it is formed by a silicon oxide film. Thegate electrode 7 is formed within the trench 5 through the gateinsulating film 6 and, for example, it is formed by a polycrystallinesilicon film with impurity introduced therein to decrease the resistancevalue thereof. The source region is constituted by an n⁺ typesemiconductor region 8 which is formed in a surface portion of the p⁻type semiconductor region 4 in contact with the region 4. The drainregion is formed by both n⁻ type semiconductor layer 1 b and n⁺ typesemiconductor layer 1 a. According to the construction of this MISFET,the source region constituted by n⁺ type semiconductor region 8, thechannel forming region constituted by p⁻ type semiconductor region, andthe drain region constituted by both n⁻ and n⁺ type semiconductor layers1 b, 1 a, are arranged in this order in the depth direction from onemain surface of the semiconductor substrate 1. Thus, the MISFET isconstituted as a vertical type wherein an electric current flows in thethickness direction of the semiconductor substrate 1. Further, it isconstituted as an n-channel conduction type wherein a channel(conduction path) of electrons is formed in the channel forming regionbetween the source region and the drain region (under the gateelectrode).

A p⁺ type semiconductor region 12 is formed in a main surface of the p⁻type semiconductor region 4. In the p⁺ type semiconductor region 12, itsthickness extending in the depth direction from one main surface of thesemiconductor substrate 1 is larger than the thickness of the n⁺ typesemiconductor region 8, and the p⁺ type semiconductor region 12 is incontact with the p⁻ type semiconductor region 4.

On one main surface of the semiconductor substrate 1 are formedinterlayer insulating films 9 so as to cover the gate electrodes 7 fromabove, the interlayer insulating films 9 being each formed by a siliconoxide film for example. Electrodes 14 and 15 (see FIG. 2) are formed of,for example, aluminum (Al) or an aluminum alloy so as to overlie theinterlayer insulating films 9. The electrode 14 is electricallyconnected to the n⁺ type semiconductor regions 8 and the p⁺ typesemiconductor regions 12 in the transistor cells 20B through connectingholes 10 formed in the interlayer insulating films 9, and is furtherconnected electrically to the n⁻ type semiconductor layer 1 b in theinteriors of Schottky cells 20A through connecting holes 10 formed inthe interlayer insulating film 9. The electrode 15 is electricallyconnected to gate lead-out wiring lines which are integral with the gateelectrodes 7.

The Schottky barrier diode in each Schottky cell 20A is formed by aSchottky junction of n⁻ type semiconductor layer 1 b and electrode 14,with n⁻ type semiconductor layer 1 b and electrode 14 being used ascathode region and anode region, respectively. Thus, the electrode 14 isused as both source electrode and anode electrode.

The width (distance from the center of one of two trenches 5 confrontingeach other to the center of the other) y of each Schottky cell 20A isset larger than the width (distance from the center of one of twotrenches 5 confronting each other to the center of the other) x of eachtransistor cell 20B. Further, a plane area of each Schottky cell 20A isset larger than that of each transistor cell 20B.

If the proportion occupied by the Schottky barrier diode SBD and thatoccupied by the power MISFET Q for one semiconductor substrate are madeconstant, the number of the trenches 5 can be decreased by making thewidth y of each Schottky cell 20A larger than the width x of eachtransistor cell 20B (width x of 20B<width y of 20A) and by therebydecreasing the number of Schottky cells 20A, so that the plane size ofthe semiconductor substrate 1 can be reduced. On the other hand, also incase of setting the width x of each transistor cell 20B larger than thewidth y of each Schottky cell 20A (width x of 20B>width y of 20A) todecrease the number of transistor cells 20B, it is possible to reducethe plane size of the semiconductor substrate 1. However, a low ONresistance is required for the power MISFET Q, and for satisfying thisrequirement it is necessary to reduce the size of each transistor cell20B and thereby enlarge the channel width per unit area. Therefore, forreducing the plane size of the semiconductor substrate 1 to attain thereduction in size of the semiconductor device, it is preferable that thenumber of Schottky cells 20A be decreased by enlarging the width y ofeach Schottky cell 20A rather than the width x of each transistor cell20B.

In a semiconductor device having both power MISFET of a trench gatestructure and Schottky barrier diode on one and the same semiconductorsubstrate, there are determined the area of the power MISFET Q and thatof the Schottky barrier diode SBD so as to satisfy characteristics ofthe power MISFET Q and VF (forward voltage) characteristics of theSchottky barrier diode SBD, which are required by a user. Therefore, itis necessary that the ratio between the area of Q and that of SBD bevaried freely to meet the user's needs. The following two methods areconceivable as methods for varying the ratio between the area of Q andthat of SBD.

According to the first method, there is used a pattern wherein Schottkycells 20A and transistor cells 20B are arranged alternately, the width yof each Schottky cell 20A and the width x of each transistor cell 20Bare set at different values, and the size of each Schottky cell 20A andthat of each transistor cell 20B are changed to change the ratio betweenthe area of the power MISFET Q and that of the Schottky barrier diodeSBD. In this method, if the sizes of each Schottky cell 20A and that ofeach transistor cell 20B are equal to each other, the proportion of thearea of Q and that of SBD become equal to each other.

According to the second method, the ratio in the number of cells betweenSchottky cells 20A and transistor cells 20B is varied to change theratio between the area of the power MISFET Q and that of the Schottkybarrier diode SBD.

In chip layout, the area of the power MISFET Q and that of the Schottkybarrier diode SBD are determined so as to satisfy characteristics of Qand that of SBD which are required by a user. In this case, the user'srequired characteristics of Q and that of SBD differ depending on thecircuit designed, so the freedom of design is necessary.

The first or the second method described above is carried out. But forthe above reason related to area efficiency it is preferable that theSchottky cells 20A be set large in width y and be arranged in a gatheredform insofar as possible. In MISFET Q2 for synchronous rectification(see FIG. 19) which requires the Schottky barrier diode SBD, a low ONresistance is needed, so the transistor cells 20B are made as small aspossible. As the cell size is reduced, the channel width (current path)per unit area increases, so it is possible to lower the ON resistance.Thus, by setting the width y of each Schottky cell 20A larger than thewidth x of each transistor cell 20B and by setting the size of theformer larger than that of the latter, the freedom of design can beensured (enhanced) in the semiconductor device wherein both powertransistor Q and Schottky barrier diode SBD are mounted on one and thesame semiconductor substrate 1.

Second Embodiment

FIG. 5 is a chip layout diagram showing a schematic construction of asemiconductor device according to a second embodiment of the presentinvention;

FIG. 6 is an enlarged, schematic plan view of region C in FIG. 5;

FIG. 7 is an enlarged, schematic sectional view of region D in FIG. 5;

FIG. 8 is a schematic sectional view taken along line B-B in FIG. 5,with an intermediate portion omitted;

FIG. 9 is a schematic sectional view taken along line C-C in FIG. 5,with an intermediate portion omitted;

FIG. 10 is a partially enlarged, schematic sectional view of FIG. 8; and

FIG. 11 is a partially enlarged, schematic sectional view of FIG. 9.

The semiconductor device of this embodiment is basically of the sameconstruction as the previous first embodiment and is different in thefollowing constructional points from the first embodiment.

In the previous first embodiment plural Schottky cells 20A are connectedin parallel to constitute one Schottky barrier diode SBD equivalently,whereas in this second embodiment, as shown in FIG. 5, one Schottkybarrier diode SBD is constituted by one Schottky cell 20A. In thissecond embodiment, moreover, a guard ring constituted by a p⁻ typesemiconductor region 2 is provided in the Schottky cell 20A, as shown inFIGS. 6 and 8. Further, in this embodiment, as shown in FIGS. 7 and 9, aguard ring constituted by a p⁻ type semiconductor region (well region) 2is provided in a peripheral portion 1N of one main surface of thesemiconductor substrate 1.

As shown in FIGS. 5 to 7, plural transistor cells 20B and one Schottkycell 20A are arranged in the cell array portion of one main surface ofthe semiconductor substrate 1. The plural transistor cells 20B aredivided into two transistor cell groups and the transistor cells 20B ineach transistor cell group are arranged so as to be spread all over inthe first direction (X direction).

The Schottky cell 20A is disposed and sandwiched in between the twotransistor cell groups. The width y of the Schottky cell 20A is set muchlarger than the width x of each transistor cell 20B. Thus, by settingthe width y of the Schottky cell 20A larger than the width x of eachtransistor cell 20B (width x of 20B<width y of 20A) and constituting asingle Schottky barrier diode SBD, it is possible to greatly decreasethe number of trenches 5 in comparison with the case where one Schottkybarrier diode SBD is constituted equivalently by plural Schottky cells20A as in the previous first embodiment, therefore it is possible tominimize the plane size of the semiconductor substrate 1. Moreparticularly, in the Schottky barrier diode SBD, the width y of eachSchottky cell 20A is set large to decrease the number of cells, while inthe power MISFET Q, the width x of each transistor cell 20B is set smallto increase the number of cells, whereby it is possible to attain a lowON resistance of the power MISFET Q and the reduction in size of thesemiconductor device.

As shown in FIGS. 6 and 7, the trenches 5 positioned between Schottkycell 20A and transistor cells 20B and the trenches 5 positioned betweentransistor cells 20B extend in the second direction (Y direction) andare rendered integral with trenches 5 which extend along the peripheralportion 1N so as to surround the cell array portion. The p⁻ typesemiconductor region 2 is formed along the trenches 5 positioned betweenSchottky cells 20A and transistor cells 20B and the trenches 5 whichextend so as to surround the cell array portion.

As shown in FIG. 8, the p⁻ type semiconductor region 2 provided in theSchottky cell 20A is formed in the n⁻ type semiconductor layer 1 b andextends in the depth direction from one main surface of thesemiconductor substrate 1 (one main surface of the n⁻ type semiconductorlayer 1 b). In Schottky cell 20A, an end portion of the Schottkyjunction between the n⁻ type semiconductor layer 1 b and the electrode14, i.e., an end portion of the barrier metal which is in contact withthe Schottky cell 20A, terminates in the p⁻ type semiconductor region 2provided in the Schottky cell 20A.

The p⁻ type semiconductor region 2 in the Schottky cell 20A is diffuseddeeper than the depth of each trench 5, and the trenches 5 positionedbetween the Schottky cell 20A and transistor cells 20B, i.e., thetrenches 5 which define the Schottky cell 20A, are each formed in the p⁻type semiconductor region 2.

Gate electrodes 7 positioned between transistor cells 20B and theSchottky cell 20A are integral with gate lead-out wiring lines 7A whichare drawn out to the Schottky cell 20A side. In the Schottky cell 20A, afield insulating film 3 thicker than the gate insulating film 6 isformed between each gate lead-out wiring line 7A and one main surface ofthe n⁻ type semiconductor layer 1 b (one main surface of thesemiconductor substrate 1). The field insulating film 3 is formedselectively by a thermal oxidation method.

As shown in FIG. 9, the p⁻ type semiconductor region 2 provided in theperipheral portion 1N is formed in the n⁻ type semiconductor layer 1 band extends in the depth direction from one main surface of thesemiconductor substrate 1. The p⁻ type semiconductor region 2 isdiffused deeper than the depth of each trench 5 and each trench 5located between the peripheral portion 1N and a transistor cell 20Badjacent thereto is formed in the p⁻ type semiconductor region 2.

Breakdown voltage (source-drain breakdown voltage) as a basicperformance of the power MISFET Q is determined by a pn junctionbreakdown voltage between the n⁻ type semiconductor layer 1 b as a drainregion and the p⁻ type semiconductor region 4 as a channel formingregion. The pn junction breakdown voltage is represented by a voltageuntil flowing of an avalanche breakdown current when a positive voltageis applied to the drain region (BVDSS state) with the gate electrode andthe source region to the ground.

When the gate electrode and the source region are connected to theground and a positive voltage is applied to the drain region into BVDSSstate, a depletion layer 19 is formed along the pn junction between thep⁻ type semiconductor region 2 and the n⁻ type semiconductor region 1 band also along the pn junction between the p⁻ type semiconductor region4 and the n⁻ type semiconductor layer 1 b. An end portion of theSchottky junction of both n⁻ type semiconductor layer 1 b and theelectrode 14 (an end portion of the barrier metal which is in contactwith the Schottky cell 20A) terminates in the p⁻ type semiconductorregion 2 provided in the Schottky cell 20A, so the electric fieldconcentrated on the end portion, indicated at 25, of the junction isrelaxed by the depletion layer 19, whereby the breakdown voltage of theSchottky barrier diode SBD can be increased.

In the Schottky cell 20A, the thick field insulating film 3 is formed onone main surface of the semiconductor substrate 1 on the p⁻ typesemiconductor region 2. By providing the field insulating film 3 in thisportion it is possible to weaken the electric field of the depletionlayer 19 which extends to the p⁻ type semiconductor region 2, so that itis possible to further raise the breakdown voltage of the Schottkybarrier diode.

The p⁻ type semiconductor region 2 in the Schottky cell 20A is diffuseddeeper than the depth of each trench 5, and a trench 5 positionedbetween the Schottky cell 20A and a transistor cell 20B, i.e., a trench5 which defines the Schottky cell 20A, is formed in the p⁻ typesemiconductor region 2. According to this construction, the depletionlayer expands to the bottom portion of the trench 5 positioned betweenthe Schottky cell 20A and the transistor cell 20B, so that voltage is nolonger applied directly to the gate insulating film 6. As a result, itis possible to lower the voltage applied to the gate insulating film 6and hence possible to raise the gate breakdown voltage of the powerMISFET Q.

In the MISFET portion, since each transistor cell undergoes avalanchebreakdown in a uniform manner, the current density does not become high(because current does not flow locally) and breakage is difficult tooccur. On the other hand, the peripheral portion 1N undergoes avalanchebreakdown near the surface of the junction, so that current flowslocally and breakage is apt to occur. For this reason, the p⁻ typesemiconductor region 2 is made deeper than the p⁻ type semiconductorregion 4 (the radius of curvature of the semiconductor region is madelarge) to make the breakdown voltage of the peripheral portion higherthan that of the MISFET portion. Further, by making the p⁻ typesemiconductor region 2 deeper than each trench 5, it is possible tofurther relax the voltage applied to the gate insulating film 6 andhence possible to raise the breakdown voltage of the peripheral portion.

For enclosing a pair of trenches 5 in the Schottky cell 20A, it isnecessary that a lateral diffusion of the p⁻ type semiconductor region 2as a guard ring be not less than the depth z of each trench 5.

Third Embodiment

FIG. 12 is a chip layout diagram showing a schematic construction of asemiconductor device according to a third embodiment of the presentinvention.

As shown in FIG. 12, the semiconductor device of this third embodimentis laid out so that a transistor device forming portion (active region)2 with a transistor cell of power MISFET formed therein is surrounded bya Schottky device forming portion 21B with Schottky barrier diode SBDformed therein. Even with such a layout, the plane size (chip size) ofthe semiconductor substrate 1 can be made small as in the firstembodiment. It is also possible to ensure the freedom in design of thesemiconductor device.

Since the Schottky barrier diode is generally a surface devicedetermined by the interface between metal and semiconductor, it ispreferable that damage in package assembly (especially damage in wirebonding) be as small as possible. As in this third embodiment, by makinglayout so that the transistor device forming portion 21A with atransistor cell of power MISFET formed therein is surrounded by theSchottky device forming portion 21B, it is possible to effect assemblyeven without wire bonding to the Schottky device forming portion 21B.

Fourth Embodiment

FIG. 13 is a schematic sectional view showing a schematic constructionof a semiconductor device according to a fourth embodiment of thepresent invention.

As shown in FIG. 13, the semiconductor device of this embodiment isbasically the same in construction as the first embodiment and isdifferent in the following constructional point.

In the Schottky cell 20A, an n⁺ type semiconductor region 22 is providedin an n⁻ type semiconductor layer 1 b, the n⁺ type semiconductor region22 having an impurity concentration higher than that of the n⁻ typesemiconductor layer 1 b. A n⁺ type semiconductor region 22 is formed ata position deeper than the metal-semiconductor interface in the Schottkycell 20A. That is, the impurity concentration of the Schottky cell 20Ais made high in its region deeper than the metal-semiconductorinterface. With such a construction, a parasitic resistance of theSchottky barrier diode can be decreased while ensuring a high breakdownvoltage.

Fifth Embodiment

FIG. 14 is a schematic sectional diagram showing a schematicconstruction of a semiconductor device according to a fifth embodimentof the present invention.

As shown in FIG. 14, the semiconductor device of this fifth embodimentis basically the same in construction as the second embodiment and isdifferent in the following constructional point.

The metal joined to the semiconductor in the Schottky cell 20A and themetal joined to the semiconductor in each transistor cell 20B aredifferent from each other, and a barrier height q*ΦB of the metaljunction in the Schottky cell 20A is larger than the barrier height q*ΦBof the metal junction in the transistor cell 20B. In this fifthembodiment, an electrode 14 formed of aluminum (Al) or an aluminum alloyfor example is joined to the Schottky cell 20A, while a metal film 13formed of titanium-tungsten (TiW) is joined to the transistor cell 20B.

In the power MISFET containing a Schottky barrier diode, an electriccurrent of several amperes is allowed to flow, so a Schottky barrierdiode having a large area is required, but there is a fear of leakagecurrent with an increase in area of the Schottky barrier diode.Therefore, using different metals, the barrier height q*ΦB of the metaljunction in the Schottky cell 20A is made larger than the barrier heightq*ΦB of the metal junction in the transistor cell 20B, whereby it ispossible to diminish the leakage current.

Generally, for electrons, the barrier height is represented as q*ΦB,where q stands for a charge quantity of electron and ΦB=ΦM−χ, wherein ΦMstands for a work function of metal and χ stands for an electronaffinity.

The barrier height q*ΦB of the metal junction in the Schottky cell 20Acan be made larger than the barrier height q*ΦB of the metal junction inthe transistor cell 20B by using a metal in the junction of the Schottkycell 20A which metal is higher in work function ΦM than the metal joinedto the transistor cell 20B. In this embodiment, Al or Al alloy is usedfor junction to the Schottky cell 20A, while TiW is used for junction tothe transistor cell 20B, the Al or Al alloy being higher in workfunction ΦM than TiW.

Sixth Embodiment

FIG. 15 is a schematic sectional diagram showing a schematicconstruction of a semiconductor device according to a sixth embodimentof the present invention.

As shown in FIG. 15, a Schottky barrier diode in a Schottky cell 20A isformed by Schottky junction of an n⁻ type semiconductor region 1 b andan electrode 14. MISFET drain region in each transistor cell 20B isconstituted by an n type semiconductor region (well region) 17 and an n⁺type semiconductor layer 1 a, the n type semiconductor region 17 beingformed in the n⁻ type semiconductor layer 1 b in contact with a p⁻ typesemiconductor region 4 as a channel forming region. The n typesemiconductor region 17 is formed at an impurity concentration lowerthan that of the n⁺ type semiconductor substrate 1 a and higher thanthat of n⁻ type semiconductor layer 1 b. That is, the MISFET drainregion is set so that the impurity concentration on the channel formingregion side is higher than that of the n⁻ type semiconductor layer 1 b.

Since the breakdown voltage of the power MISFET is a pn junctionbreakdown voltage between the p⁻ type semiconductor region 4 as thechannel forming region and the drain region, a depletion layer extendsto both p and n type regions. On the other hand, since the breakdownvoltage of the Schottky barrier diode is a Schottky junction breakdownvoltage between metal and n type cathode region, a depletion layerextends to only the n type cathode region. Therefore, if both powerMISFET and Schottky barrier diode are formed in n type regions of thesame impurity concentration, the latter is sure to become lower inbreakdown voltage.

If the breakdown voltage of the Schottky barrier diode is lower thanthat of the power MISFET, there always occurs breakdown in the Schottkybarrier diode, thus resulting in deterioration of the reliability. Inthe case where the power MISFET is lower in breakdown voltage, thereoccurs breakdown in the pn junction within the bulk, a variation ofcharacteristic is difficult to occur. On the other hand, if the Schottkybarrier diode is lower in breakdown voltage, since it is an interfacedevice, a variation of characteristic is apt to occur due to carriersgenerated upon breakdown.

Such a problem can be solved by making the n type cathode region in theSchottky barrier diode lower in impurity concentration than the n typedrain region in MISFET. With such a construction, it is possible to makethe breakdown voltage of the power MISFET low and that the Schottkybarrier diode high.

Although in this embodiment reference has been made to an example ofapplying the present invention to the semiconductor device having thepower MISFET of a trench gate structure and the Schottky barrier diode,the present invention is also applicable to a semiconductor devicehaving a power MISFET of a planar structure and a Schottky barrier diodeand a semiconductor device having a power MISFET of an LD (LateralDouble Diffusion Self-aligned) structure and a Schottky barrier diode.

Seventh Embodiment

FIG. 16 is a schematic sectional diagram showing a schematicconstruction of a semiconductor device according to a seventh embodimentof the present invention.

As shown in FIG. 16, a metal junction in a Schottky barrier diode in aSchottky cell 20A is carried out at a bottom portion of a trench 18formed in one main surface of a semiconductor substrate 1. An n⁻ typesemiconductor layer 1 b with phosphorus (P) introduced therein becomeshigher in density than in the initial state due to surface-segregationof phosphorus in the thermal oxidation step. The trench 18 is formed bydigging down the surface portion which has become high in density by thesegregation, and in the interior bottom portion of the trench 18 thereis performed metal junction of the Schottky barrier diode in theSchottky cell 20A, whereby the Schottky barrier diode can be made higherin breakdown voltage.

Eighth Embodiment

In this embodiment a description will be given below of an example inwhich the present invention is applied to a semiconductor device havinga power MISFET of a planar structure and a Schottky barrier diode.

FIG. 17 is a schematic sectional diagram showing a schematicconstruction of a semiconductor device according to an eighth embodimentof the present invention.

As shown in FIG. 17, the semiconductor device of this eighth embodimentis of basically the same construction as the sixth embodiment and isdifferent in the following constructional point.

In a transistor cell 20B, a MISFET has a gate electrode 7 which isdisposed on a main surface of a semiconductor substrate 1 (main surfaceof an n⁻ type semiconductor layer 1 b) through a gate insulating film 6.

Also in such a semiconductor device having a power MISFET of a planarstructure and a Schottky barrier diode, by making the n type cathoderegion in the Schottky barrier diode lower in impurity concentrationthan the n type drain region in the MISFET as in the sixth embodiment,it is possible to make the breakdown voltage of the power MISFET low andthat of the Schottky barrier diode high.

Ninth Embodiment

In this embodiment, an example of applying the present invention to asemiconductor device having a power MISFET of a lateral double diffusionself-aligned structure and a Schottky barrier diode will be described.

FIG. 18 is a schematic sectional diagram showing a schematicconstruction of a semiconductor device according to a ninth embodimentof the present invention.

As shown in FIG. 18, a Schottky barrier diode in a Schottky cell isformed by Schottky junction of an n⁻ type semiconductor layer lb and anelectrode 24A. A MISFET in a transistor cell has a lateral structurewherein an electric current flows in a surface direction of asemiconductor substrate 1.

The MISFET in the transistor cell mainly comprises a channel formingregion, a gate insulating film 6, a gate electrode 7, a source region,and a drain region. The channel forming region is formed by a p⁻ typesemiconductor region 4 provided on a main surface of an n⁻ typesemiconductor layer 1 b. The gate insulating film 6 is formed on themain surface of the n⁻ type semiconductor layer 1 b in opposition to thechannel forming region. The gate electrode 7 is formed on the mainsurface of the n⁻ type semiconductor layer 1 b through the gateinsulating film 6. The source region is formed by an n⁺ typesemiconductor region 8 which is formed in a surface portion of the p⁻type semiconductor region 4 in contact with the region 4. The drainregion is composed of the n type semiconductor region 17 which isprovided in the n⁻ type semiconductor layer 1 b in contact with the p−type semiconductor region 4 as a channel forming region and an n⁺ typesemiconductor region 23 provided in the n type semiconductor region 17spacedly from the p⁻ type semiconductor region 4. The n typesemiconductor region 17 is formed at an impurity concentration lowerthan that of the n⁺ type semiconductor region 23 and higher than that ofthe n⁻ type semiconductor layer 1 b. Thus, also in the MISFET of thisembodiment, the impurity concentration on the channel forming regionside of the drain region is set higher than that of the n⁻ typesemiconductor layer 1 b.

A p⁺ type semiconductor region 12 is formed in a main surface of the p⁻type semiconductor region 4, and a source electrode 24B is electricallyconnected to both p⁺ type semiconductor region 12 and n⁺ typesemiconductor region 8 through a connecting hole formed in an interlayerinsulating film 9. Thus, in the MISFET in each transistor cell, thesource region and the channel forming region are fixed to the samepotential.

A drain electrode 24C is electrically connected to the n³⁰ typesemiconductor region 23 through a connecting hole formed in theinterlayer insulating film 9. In the Schottky cell, an electrode 24A iselectrically connected to the n⁻ type semiconductor layer 1 b through aconnecting hole formed in the interlayer insulating film 9.

The Schottky barrier diode in the Schottky cell is formed by Schottkyconnection between the n⁻ type semiconductor layer 1 b and the electrode24A. On the other hand, the transistor cell MISFET is higher in impurityconcentration on the channel forming side of its drain region than then⁻ type semiconductor layer 1 b. That is, in this embodiment, thecathode region in the Schottky barrier diode is lower in impurityconcentration than the channel forming region side of the MISFET drainregion, so the breakdown voltage of the Schottky barrier diode can bemade higher than that of the power MISFET as in the sixth embodiment.

Although in each of the semiconductor devices described in the abovefirst to fifth embodiments both power MISFET of a trench gate structureand Schottky barrier diode are mounted on one and the same substrate,the invention carried out in the first to fifth embodiments is alsoapplicable to a semiconductor device having a power MISFET of a planarstructure and a Schottky barrier diode and a semiconductor device havinga power MISFET of lateral double diffusion self-aligned structure and aSchottky barrier diode.

Although the present invention has been described above concretely onthe basis of the above embodiments, it goes without saying that theinvention is not limited to those embodiments, but that various changesmay be made within the scope not departing from the gist of theinvention.

The following is a brief description of effects obtained by typicalinventions disclosed herein.

According to the present invention it is possible to attain thereduction in size of a semiconductor device having both power transistorand Schottky harrier diode on one and the same semiconductor substrate.

According to the present invention it is possible to ensure the freedomof design in a semiconductor device having both power transistor andSchottky barrier diode on one and the same semiconductor substrate.

According to the present invention it is possible to enhance thebreakdown voltage of a Schottky barrier diode in a semiconductor devicehaving both power transistor and Schottky barrier diode on one and thesame semiconductor substrate.

According to the present invention it is possible to enhance thebreakdown voltage (source-drain breakdown voltage) of a power transistorin a semiconductor device having both power transistor and Schottkybarrier diode on one and the same semiconductor substrate.

1. A semiconductor device including a MOSFET, comprising: asemiconductor substrate having a main surface and a back surfaceopposite to the main surface; a plurality of transistor cell regionsdisposed in the semiconductor substrate, each of the plurality oftransistor cell regions including: a plurality of trenches disposed onthe main surface of the semiconductor substrate, a well region in whicha channel of the MOSFET is formed, disposed between the plurality oftrenches, a gate insulating film of the MOSFET disposed in each of theplurality of trenches, a gate electrode of the MOSFET disposed over thegate insulating film in each of the plurality of trenches, and a sourceregion of the MOSFET disposed in the well region; a source electrode ofthe MOSFET disposed over the main surface of the semiconductor substrateand being in contact with a top surface of the source region in each ofthe plurality of transistor cell regions, a drain electrode of theMOSFET disposed over the back surface of the semiconductor substrate andelectrically connected to the semiconductor substrate; and a Schottkycell region disposed between the plurality of transistor cell regions inthe semiconductor substrate, the source electrode being in contact witha part of the main surface of the semiconductor substrate so as to forma Schottky junction in the Schottky cell region, and the Schottkyjunction being located lower than a top surface of the source region. 2.A semiconductor device according to claim 1, wherein the semiconductorsubstrate and the source region have a first conductivity type, the wellregion has a second conductivity type which is opposite to the firstconductivity type.
 3. A semiconductor device according to claim 2,wherein the first and second conductivity types are n-type and p-type,respectively.
 4. A semiconductor device according to claim 3, whereinthe semiconductor substrate includes phosphorous.
 5. A semiconductordevice according to claim 1, wherein a plurality of field insulatingfilms are disposed on the main surface of the semiconductor substrate inthe Schottky cell region, the source electrode is partially located overthe field insulating films, and the Schottky junction is formed betweenthe field insulating films.
 6. A semiconductor device according to claim1, wherein the source electrode is comprised of aluminum.